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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_regex.syntax.basic_extended"></a><a class="link" href="basic_extended.html" title="POSIX Extended Regular Expression Syntax">POSIX Extended Regular
      Expression Syntax</a>
</h3></div></div></div>
<h4>
<a name="boost_regex.syntax.basic_extended.h0"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.synopsis"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.synopsis">Synopsis</a>
      </h4>
<p>
        The POSIX-Extended regular expression syntax is supported by the POSIX C
        regular expression API's, and variations are used by the utilities <code class="computeroutput"><span class="identifier">egrep</span></code> and <code class="computeroutput"><span class="identifier">awk</span></code>.
        You can construct POSIX extended regular expressions in Boost.Regex by passing
        the flag <code class="computeroutput"><span class="identifier">extended</span></code> to the
        regex constructor, for example:
      </p>
<pre class="programlisting"><span class="comment">// e1 is a case sensitive POSIX-Extended expression:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e1</span><span class="special">(</span><span class="identifier">my_expression</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">extended</span><span class="special">);</span>
<span class="comment">// e2 a case insensitive POSIX-Extended expression:</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e2</span><span class="special">(</span><span class="identifier">my_expression</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">extended</span><span class="special">|</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">icase</span><span class="special">);</span>
</pre>
<a name="boost_regex.posix_extended_syntax"></a><h4>
<a name="boost_regex.syntax.basic_extended.h1"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.posix_extended_syntax"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.posix_extended_syntax">POSIX Extended
        Syntax</a>
      </h4>
<p>
        In POSIX-Extended regular expressions, all characters match themselves except
        for the following special characters:
      </p>
<pre class="programlisting">.[{}()\*+?|^$</pre>
<h5>
<a name="boost_regex.syntax.basic_extended.h2"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.wildcard"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.wildcard">Wildcard:</a>
      </h5>
<p>
        The single character '.' when used outside of a character set will match
        any single character except:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            The NULL character when the flag <code class="computeroutput"><span class="identifier">match_no_dot_null</span></code>
            is passed to the matching algorithms.
          </li>
<li class="listitem">
            The newline character when the flag <code class="computeroutput"><span class="identifier">match_not_dot_newline</span></code>
            is passed to the matching algorithms.
          </li>
</ul></div>
<h5>
<a name="boost_regex.syntax.basic_extended.h3"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.anchors"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.anchors">Anchors:</a>
      </h5>
<p>
        A '^' character shall match the start of a line when used as the first character
        of an expression, or the first character of a sub-expression.
      </p>
<p>
        A '$' character shall match the end of a line when used as the last character
        of an expression, or the last character of a sub-expression.
      </p>
<h5>
<a name="boost_regex.syntax.basic_extended.h4"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.marked_sub_expressions"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.marked_sub_expressions">Marked
        sub-expressions:</a>
      </h5>
<p>
        A section beginning <code class="computeroutput"><span class="special">(</span></code> and ending
        <code class="computeroutput"><span class="special">)</span></code> acts as a marked sub-expression.
        Whatever matched the sub-expression is split out in a separate field by the
        matching algorithms. Marked sub-expressions can also repeated, or referred
        to by a back-reference.
      </p>
<h5>
<a name="boost_regex.syntax.basic_extended.h5"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.repeats"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.repeats">Repeats:</a>
      </h5>
<p>
        Any atom (a single character, a marked sub-expression, or a character class)
        can be repeated with the <code class="computeroutput"><span class="special">*</span></code>,
        <code class="computeroutput"><span class="special">+</span></code>, <code class="computeroutput"><span class="special">?</span></code>,
        and <code class="computeroutput"><span class="special">{}</span></code> operators.
      </p>
<p>
        The <code class="computeroutput"><span class="special">*</span></code> operator will match the
        preceding atom <span class="emphasis"><em>zero or more times</em></span>, for example the expression
        <code class="computeroutput"><span class="identifier">a</span><span class="special">*</span><span class="identifier">b</span></code> will match any of the following:
      </p>
<pre class="programlisting">b
ab
aaaaaaaab
</pre>
<p>
        The <code class="computeroutput"><span class="special">+</span></code> operator will match the
        preceding atom <span class="emphasis"><em>one or more times</em></span>, for example the expression
        a+b will match any of the following:
      </p>
<pre class="programlisting">ab
aaaaaaaab
</pre>
<p>
        But will not match:
      </p>
<pre class="programlisting">b
</pre>
<p>
        The <code class="computeroutput"><span class="special">?</span></code> operator will match the
        preceding atom <span class="emphasis"><em>zero or one times</em></span>, for example the expression
        <code class="computeroutput"><span class="identifier">ca</span><span class="special">?</span><span class="identifier">b</span></code> will match any of the following:
      </p>
<pre class="programlisting">cb
cab
</pre>
<p>
        But will not match:
      </p>
<pre class="programlisting">caab
</pre>
<p>
        An atom can also be repeated with a bounded repeat:
      </p>
<p>
        <code class="computeroutput"><span class="identifier">a</span><span class="special">{</span><span class="identifier">n</span><span class="special">}</span></code> Matches
        'a' repeated <span class="emphasis"><em>exactly n times</em></span>.
      </p>
<p>
        <code class="computeroutput"><span class="identifier">a</span><span class="special">{</span><span class="identifier">n</span><span class="special">,}</span></code> Matches
        'a' repeated <span class="emphasis"><em>n or more times</em></span>.
      </p>
<p>
        <code class="computeroutput"><span class="identifier">a</span><span class="special">{</span><span class="identifier">n</span><span class="special">,</span> <span class="identifier">m</span><span class="special">}</span></code> Matches 'a' repeated <span class="emphasis"><em>between n
        and m times inclusive</em></span>.
      </p>
<p>
        For example:
      </p>
<pre class="programlisting">^a{2,3}$</pre>
<p>
        Will match either of:
      </p>
<pre class="programlisting"><span class="identifier">aa</span>
<span class="identifier">aaa</span>
</pre>
<p>
        But neither of:
      </p>
<pre class="programlisting"><span class="identifier">a</span>
<span class="identifier">aaaa</span>
</pre>
<p>
        It is an error to use a repeat operator, if the preceding construct can not
        be repeated, for example:
      </p>
<pre class="programlisting"><span class="identifier">a</span><span class="special">(*)</span>
</pre>
<p>
        Will raise an error, as there is nothing for the <code class="computeroutput"><span class="special">*</span></code>
        operator to be applied to.
      </p>
<h5>
<a name="boost_regex.syntax.basic_extended.h6"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.back_references"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.back_references">Back
        references:</a>
      </h5>
<p>
        An escape character followed by a digit <span class="emphasis"><em>n</em></span>, where <span class="emphasis"><em>n</em></span>
        is in the range 1-9, matches the same string that was matched by sub-expression
        <span class="emphasis"><em>n</em></span>. For example the expression:
      </p>
<pre class="programlisting">^(a*)[^a]*\1$</pre>
<p>
        Will match the string:
      </p>
<pre class="programlisting"><span class="identifier">aaabbaaa</span>
</pre>
<p>
        But not the string:
      </p>
<pre class="programlisting"><span class="identifier">aaabba</span>
</pre>
<div class="caution"><table border="0" summary="Caution">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../doc/src/images/caution.png"></td>
<th align="left">Caution</th>
</tr>
<tr><td align="left" valign="top"><p>
          The POSIX standard does not support back-references for "extended"
          regular expressions, this is a compatible extension to that standard.
        </p></td></tr>
</table></div>
<h5>
<a name="boost_regex.syntax.basic_extended.h7"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.alternation"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.alternation">Alternation</a>
      </h5>
<p>
        The <code class="computeroutput"><span class="special">|</span></code> operator will match either
        of its arguments, so for example: <code class="computeroutput"><span class="identifier">abc</span><span class="special">|</span><span class="identifier">def</span></code> will
        match either "abc" or "def".
      </p>
<p>
        Parenthesis can be used to group alternations, for example: <code class="computeroutput"><span class="identifier">ab</span><span class="special">(</span><span class="identifier">d</span><span class="special">|</span><span class="identifier">ef</span><span class="special">)</span></code>
        will match either of "abd" or "abef".
      </p>
<h5>
<a name="boost_regex.syntax.basic_extended.h8"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.character_sets"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.character_sets">Character
        sets:</a>
      </h5>
<p>
        A character set is a bracket-expression starting with <code class="literal">[</code>
        and ending with <code class="literal">]</code>, it defines a set of characters, and
        matches any single character that is a member of that set.
      </p>
<p>
        A bracket expression may contain any combination of the following:
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h9"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.single_characters"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.single_characters">Single
        characters:</a>
      </h6>
<p>
        For example <code class="computeroutput"><span class="special">[</span><span class="identifier">abc</span><span class="special">]</span></code>, will match any of the characters 'a', 'b',
        or 'c'.
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h10"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.character_ranges"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.character_ranges">Character
        ranges:</a>
      </h6>
<p>
        For example <code class="computeroutput"><span class="special">[</span><span class="identifier">a</span><span class="special">-</span><span class="identifier">c</span><span class="special">]</span></code>
        will match any single character in the range 'a' to 'c'. By default, for
        POSIX-Extended regular expressions, a character <span class="emphasis"><em>x</em></span> is
        within the range <span class="emphasis"><em>y</em></span> to <span class="emphasis"><em>z</em></span>, if it
        collates within that range; this results in locale specific behavior . This
        behavior can be turned off by unsetting the <code class="computeroutput"><span class="identifier">collate</span></code>
        <a class="link" href="../ref/syntax_option_type.html" title="syntax_option_type">option flag</a> - in
        which case whether a character appears within a range is determined by comparing
        the code points of the characters only.
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h11"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.negation"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.negation">Negation:</a>
      </h6>
<p>
        If the bracket-expression begins with the ^ character, then it matches the
        complement of the characters it contains, for example <code class="computeroutput"><span class="special">[^</span><span class="identifier">a</span><span class="special">-</span><span class="identifier">c</span><span class="special">]</span></code> matches any character that is not in the
        range <code class="computeroutput"><span class="identifier">a</span><span class="special">-</span><span class="identifier">c</span></code>.
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h12"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.character_classes"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.character_classes">Character
        classes:</a>
      </h6>
<p>
        An expression of the form <code class="computeroutput"><span class="special">[[:</span><span class="identifier">name</span><span class="special">:]]</span></code>
        matches the named character class "name", for example <code class="computeroutput"><span class="special">[[:</span><span class="identifier">lower</span><span class="special">:]]</span></code> matches any lower case character. See
        <a class="link" href="character_classes.html" title="Character Class Names">character class names</a>.
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h13"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.collating_elements"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.collating_elements">Collating
        Elements:</a>
      </h6>
<p>
        An expression of the form <code class="computeroutput"><span class="special">[[.</span><span class="identifier">col</span><span class="special">.]</span></code> matches
        the collating element <span class="emphasis"><em>col</em></span>. A collating element is any
        single character, or any sequence of characters that collates as a single
        unit. Collating elements may also be used as the end point of a range, for
        example: <code class="computeroutput"><span class="special">[[.</span><span class="identifier">ae</span><span class="special">.]-</span><span class="identifier">c</span><span class="special">]</span></code>
        matches the character sequence "ae", plus any single character
        in the range "ae"-c, assuming that "ae" is treated as
        a single collating element in the current locale.
      </p>
<p>
        Collating elements may be used in place of escapes (which are not normally
        allowed inside character sets), for example <code class="computeroutput"><span class="special">[[.^.]</span><span class="identifier">abc</span><span class="special">]</span></code> would
        match either one of the characters 'abc^'.
      </p>
<p>
        As an extension, a collating element may also be specified via its <a class="link" href="collating_names.html" title="Collating Names">symbolic name</a>, for example:
      </p>
<pre class="programlisting"><span class="special">[[.</span><span class="identifier">NUL</span><span class="special">.]]</span>
</pre>
<p>
        matches a NUL character.
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h14"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.equivalence_classes"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.equivalence_classes">Equivalence
        classes:</a>
      </h6>
<p>
        An expression of the form <code class="computeroutput"><span class="special">[[=</span><span class="identifier">col</span><span class="special">=]]</span></code>,
        matches any character or collating element whose primary sort key is the
        same as that for collating element <span class="emphasis"><em>col</em></span>, as with collating
        elements the name <span class="emphasis"><em>col</em></span> may be a <a class="link" href="collating_names.html" title="Collating Names">symbolic
        name</a>. A primary sort key is one that ignores case, accentation, or
        locale-specific tailorings; so for example <code class="computeroutput"><span class="special">[[=</span><span class="identifier">a</span><span class="special">=]]</span></code> matches
        any of the characters: a, À, Á, Â, Ã, Ä, Å, A, à, á, â, ã, ä and å. Unfortunately implementation
        of this is reliant on the platform's collation and localisation support;
        this feature can not be relied upon to work portably across all platforms,
        or even all locales on one platform.
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h15"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.combinations"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.combinations">Combinations:</a>
      </h6>
<p>
        All of the above can be combined in one character set declaration, for example:
        <code class="computeroutput"><span class="special">[[:</span><span class="identifier">digit</span><span class="special">:]</span><span class="identifier">a</span><span class="special">-</span><span class="identifier">c</span><span class="special">[.</span><span class="identifier">NUL</span><span class="special">.]]</span></code>.
      </p>
<h5>
<a name="boost_regex.syntax.basic_extended.h16"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.escapes"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.escapes">Escapes</a>
      </h5>
<p>
        The POSIX standard defines no escape sequences for POSIX-Extended regular
        expressions, except that:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            Any special character preceded by an escape shall match itself.
          </li>
<li class="listitem">
            The effect of any ordinary character being preceded by an escape is undefined.
          </li>
<li class="listitem">
            An escape inside a character class declaration shall match itself: in
            other words the escape character is not "special" inside a
            character class declaration; so <code class="computeroutput"><span class="special">[\^]</span></code>
            will match either a literal '\' or a '^'.
          </li>
</ul></div>
<p>
        However, that's rather restrictive, so the following standard-compatible
        extensions are also supported by Boost.Regex:
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h17"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.escapes_matching_a_specific_char"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.escapes_matching_a_specific_char">Escapes
        matching a specific character</a>
      </h6>
<p>
        The following escape sequences are all synonyms for single characters:
      </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Escape
                </p>
              </th>
<th>
                <p>
                  Character
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  \a
                </p>
              </td>
<td>
                <p>
                  '\a'
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \e
                </p>
              </td>
<td>
                <p>
                  0x1B
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \f
                </p>
              </td>
<td>
                <p>
                  \f
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \n
                </p>
              </td>
<td>
                <p>
                  \n
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \r
                </p>
              </td>
<td>
                <p>
                  \r
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \t
                </p>
              </td>
<td>
                <p>
                  \t
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \v
                </p>
              </td>
<td>
                <p>
                  \v
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \b
                </p>
              </td>
<td>
                <p>
                  \b (but only inside a character class declaration).
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \cX
                </p>
              </td>
<td>
                <p>
                  An ASCII escape sequence - the character whose code point is X
                  % 32
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \xdd
                </p>
              </td>
<td>
                <p>
                  A hexadecimal escape sequence - matches the single character whose
                  code point is 0xdd.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \x{dddd}
                </p>
              </td>
<td>
                <p>
                  A hexadecimal escape sequence - matches the single character whose
                  code point is 0xdddd.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \0ddd
                </p>
              </td>
<td>
                <p>
                  An octal escape sequence - matches the single character whose code
                  point is 0ddd.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \N{Name}
                </p>
              </td>
<td>
                <p>
                  Matches the single character which has the symbolic name <span class="emphasis"><em>Name</em></span>.
                  For example <code class="computeroutput"><span class="special">\\</span><span class="identifier">N</span><span class="special">{</span><span class="identifier">newline</span><span class="special">}</span></code> matches the single character \n.
                </p>
              </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="boost_regex.syntax.basic_extended.h18"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.single_character_character_class"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.single_character_character_class">"Single
        character" character classes:</a>
      </h6>
<p>
        Any escaped character <span class="emphasis"><em>x</em></span>, if <span class="emphasis"><em>x</em></span> is
        the name of a character class shall match any character that is a member
        of that class, and any escaped character <span class="emphasis"><em>X</em></span>, if <span class="emphasis"><em>x</em></span>
        is the name of a character class, shall match any character not in that class.
      </p>
<p>
        The following are supported by default:
      </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Escape sequence
                </p>
              </th>
<th>
                <p>
                  Equivalent to
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">d</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">digit</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">l</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">lower</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">s</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">space</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">u</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">upper</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">w</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">word</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">D</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">digit</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">L</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">lower</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">S</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">space</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">U</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">upper</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">W</span></code>
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">word</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="boost_regex.syntax.basic_extended.h19"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.character_properties"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.character_properties">Character
        Properties</a>
      </h6>
<p>
        The character property names in the following table are all equivalent to
        the names used in character classes.
      </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Form
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
<th>
                <p>
                  Equivalent character set form
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">pX</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches any character that has the property X.
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">X</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">p</span><span class="special">{</span><span class="identifier">Name</span><span class="special">}</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches any character that has the property Name.
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[[:</span><span class="identifier">Name</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">PX</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches any character that does not have the property X.
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">X</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">P</span><span class="special">{</span><span class="identifier">Name</span><span class="special">}</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches any character that does not have the property Name.
                </p>
              </td>
<td>
                <p>
                  <code class="computeroutput"><span class="special">[^[:</span><span class="identifier">Name</span><span class="special">:]]</span></code>
                </p>
              </td>
</tr>
</tbody>
</table></div>
<p>
        For example <code class="computeroutput"><span class="special">\</span><span class="identifier">pd</span></code>
        matches any "digit" character, as does <code class="computeroutput"><span class="special">\</span><span class="identifier">p</span><span class="special">{</span><span class="identifier">digit</span><span class="special">}</span></code>.
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h20"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.word_boundaries"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.word_boundaries">Word
        Boundaries</a>
      </h6>
<p>
        The following escape sequences match the boundaries of words:
      </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Escape
                </p>
              </th>
<th>
                <p>
                  Meaning
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\&lt;</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches the start of a word.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\&gt;</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches the end of a word.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">b</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches a word boundary (the start or end of a word).
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">B</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches only when not at a word boundary.
                </p>
              </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="boost_regex.syntax.basic_extended.h21"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.buffer_boundaries"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.buffer_boundaries">Buffer
        boundaries</a>
      </h6>
<p>
        The following match only at buffer boundaries: a "buffer" in this
        context is the whole of the input text that is being matched against (note
        that ^ and $ may match embedded newlines within the text).
      </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Escape
                </p>
              </th>
<th>
                <p>
                  Meaning
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  \`
                </p>
              </td>
<td>
                <p>
                  Matches at the start of a buffer only.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  \'
                </p>
              </td>
<td>
                <p>
                  Matches at the end of a buffer only.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">A</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches at the start of a buffer only (the same as \`).
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">z</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches at the end of a buffer only (the same as \').
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">Z</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches an optional sequence of newlines at the end of a buffer:
                  equivalent to the regular expression <code class="computeroutput"><span class="special">\</span><span class="identifier">n</span><span class="special">*\</span><span class="identifier">z</span></code>
                </p>
              </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="boost_regex.syntax.basic_extended.h22"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.continuation_escape"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.continuation_escape">Continuation
        Escape</a>
      </h6>
<p>
        The sequence <code class="computeroutput"><span class="special">\</span><span class="identifier">G</span></code>
        matches only at the end of the last match found, or at the start of the text
        being matched if no previous match was found. This escape useful if you're
        iterating over the matches contained within a text, and you want each subsequence
        match to start where the last one ended.
      </p>
<h6>
<a name="boost_regex.syntax.basic_extended.h23"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.quoting_escape"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.quoting_escape">Quoting
        escape</a>
      </h6>
<p>
        The escape sequence <code class="computeroutput"><span class="special">\</span><span class="identifier">Q</span></code>
        begins a "quoted sequence": all the subsequent characters are treated
        as literals, until either the end of the regular expression or <code class="computeroutput"><span class="special">\</span><span class="identifier">E</span></code> is found.
        For example the expression: <code class="computeroutput"><span class="special">\</span><span class="identifier">Q</span><span class="special">\*+\</span><span class="identifier">Ea</span><span class="special">+</span></code> would match either of:
      </p>
<pre class="programlisting"><span class="special">\*+</span><span class="identifier">a</span>
<span class="special">\*+</span><span class="identifier">aaa</span>
</pre>
<h6>
<a name="boost_regex.syntax.basic_extended.h24"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.unicode_escapes"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.unicode_escapes">Unicode
        escapes</a>
      </h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Escape
                </p>
              </th>
<th>
                <p>
                  Meaning
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">C</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches a single code point: in Boost regex this has exactly the
                  same effect as a "." operator.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="special">\</span><span class="identifier">X</span></code>
                </p>
              </td>
<td>
                <p>
                  Matches a combining character sequence: that is any non-combining
                  character followed by a sequence of zero or more combining characters.
                </p>
              </td>
</tr>
</tbody>
</table></div>
<h6>
<a name="boost_regex.syntax.basic_extended.h25"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.any_other_escape"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.any_other_escape">Any
        other escape</a>
      </h6>
<p>
        Any other escape sequence matches the character that is escaped, for example
        \@ matches a literal '@'.
      </p>
<h5>
<a name="boost_regex.syntax.basic_extended.h26"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.operator_precedence"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.operator_precedence">Operator
        precedence</a>
      </h5>
<p>
        The order of precedence for of operators is as follows:
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            Collation-related bracket symbols <code class="computeroutput"><span class="special">[==]</span>
            <span class="special">[::]</span> <span class="special">[..]</span></code>
          </li>
<li class="listitem">
            Escaped characters <code class="computeroutput"><span class="special">\</span></code>
          </li>
<li class="listitem">
            Character set (bracket expression) <code class="computeroutput"><span class="special">[]</span></code>
          </li>
<li class="listitem">
            Grouping <code class="computeroutput"><span class="special">()</span></code>
          </li>
<li class="listitem">
            Single-character-ERE duplication <code class="computeroutput"><span class="special">*</span>
            <span class="special">+</span> <span class="special">?</span>
            <span class="special">{</span><span class="identifier">m</span><span class="special">,</span><span class="identifier">n</span><span class="special">}</span></code>
          </li>
<li class="listitem">
            Concatenation
          </li>
<li class="listitem">
            Anchoring ^$
          </li>
<li class="listitem">
            Alternation <code class="computeroutput"><span class="special">|</span></code>
          </li>
</ol></div>
<h5>
<a name="boost_regex.syntax.basic_extended.h27"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.what_gets_matched"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.what_gets_matched">What
        Gets Matched</a>
      </h5>
<p>
        When there is more that one way to match a regular expression, the "best"
        possible match is obtained using the <a class="link" href="leftmost_longest_rule.html" title="The Leftmost Longest Rule">leftmost-longest
        rule</a>.
      </p>
<h4>
<a name="boost_regex.syntax.basic_extended.h28"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.variations"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.variations">Variations</a>
      </h4>
<h5>
<a name="boost_regex.syntax.basic_extended.h29"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.egrep"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.egrep">Egrep</a>
      </h5>
<p>
        When an expression is compiled with the <a class="link" href="../ref/syntax_option_type.html" title="syntax_option_type">flag
        <code class="computeroutput"><span class="identifier">egrep</span></code></a> set, then the
        expression is treated as a newline separated list of <a class="link" href="basic_extended.html#boost_regex.posix_extended_syntax">POSIX-Extended
        expressions</a>, a match is found if any of the expressions in the list
        match, for example:
      </p>
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span> <span class="identifier">e</span><span class="special">(</span><span class="string">"abc\ndef"</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">regex</span><span class="special">::</span><span class="identifier">egrep</span><span class="special">);</span>
</pre>
<p>
        will match either of the POSIX-Basic expressions "abc" or "def".
      </p>
<p>
        As its name suggests, this behavior is consistent with the Unix utility
        <code class="computeroutput"><span class="identifier">egrep</span></code>, and with grep when
        used with the -E option.
      </p>
<h5>
<a name="boost_regex.syntax.basic_extended.h30"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.awk"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.awk">awk</a>
      </h5>
<p>
        In addition to the <a class="link" href="basic_extended.html#boost_regex.posix_extended_syntax">POSIX-Extended
        features</a> the escape character is special inside a character class
        declaration.
      </p>
<p>
        In addition, some escape sequences that are not defined as part of POSIX-Extended
        specification are required to be supported - however Boost.Regex supports
        these by default anyway.
      </p>
<h4>
<a name="boost_regex.syntax.basic_extended.h31"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.options"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.options">Options</a>
      </h4>
<p>
        There are a <a class="link" href="../ref/syntax_option_type/syntax_option_type_extended.html" title="Options for POSIX Extended Regular Expressions">variety
        of flags</a> that may be combined with the <code class="computeroutput"><span class="identifier">extended</span></code>
        and <code class="computeroutput"><span class="identifier">egrep</span></code> options when constructing
        the regular expression, in particular note that the <a class="link" href="../ref/syntax_option_type/syntax_option_type_extended.html" title="Options for POSIX Extended Regular Expressions"><code class="computeroutput"><span class="identifier">newline_alt</span></code></a> option alters the syntax,
        while the <a class="link" href="../ref/syntax_option_type/syntax_option_type_extended.html" title="Options for POSIX Extended Regular Expressions"><code class="computeroutput"><span class="identifier">collate</span></code>, <code class="computeroutput"><span class="identifier">nosubs</span></code>
        and <code class="computeroutput"><span class="identifier">icase</span></code> options</a>
        modify how the case and locale sensitivity are to be applied.
      </p>
<h4>
<a name="boost_regex.syntax.basic_extended.h32"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_extended.references"></a></span><a class="link" href="basic_extended.html#boost_regex.syntax.basic_extended.references">References</a>
      </h4>
<p>
        <a href="http://www.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap09.html" target="_top">IEEE
        Std 1003.1-2001, Portable Operating System Interface (POSIX ), Base Definitions
        and Headers, Section 9, Regular Expressions.</a>
      </p>
<p>
        <a href="http://www.opengroup.org/onlinepubs/000095399/utilities/grep.html" target="_top">IEEE
        Std 1003.1-2001, Portable Operating System Interface (POSIX ), Shells and
        Utilities, Section 4, Utilities, egrep.</a>
      </p>
<p>
        <a href="http://www.opengroup.org/onlinepubs/000095399/utilities/awk.html" target="_top">IEEE
        Std 1003.1-2001, Portable Operating System Interface (POSIX ), Shells and
        Utilities, Section 4, Utilities, awk.</a>
      </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 1998-2013 John Maddock<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></td>
</tr></table>
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